Reactor is one of the key equipments in the chemical production process, due to the complexity of the chemical reaction mechanism inside the reactor, the external conditions, changes in the amount of raw materials added, changes in the temperature of the heating and cooling water, etc. have a large impact on the system, so that the system itself has a large time-varying, non-linear and time-lagged, and is therefore subject to the attention of the Institute of the field of process control. Combined with the current advanced automation technology at home and abroad, the control program of mass metering is proposed in the feed control of the reactor, and the ideal control effect has been achieved.

The main proportioning of the dosing control system realizes the measurement of the quality of raw materials through the gear flow meter, temperature compensation and density calibration, and a cut-off valve is set up at each pipeline into the reactor to interlock with the flow rate, and the flow meter of each reactor adopts the 4~20mA standard signal to enter into the PLC in the control room, and the PLC in the control room is controlled in accordance with the set proportion of the formula. Realize the automatic metering of the feed. The study found that the original control program has the following disadvantages:

(1) Due to the transport of raw materials need steam accompanied by heat (raw materials with a low condensation point), the transport pipeline is long and contains large solid particles, resulting in unstable density of the liquid flowing through the flowmeter, the flowmeter internal components of the wear rate is large, and it is impossible to achieve the process of the 0.1% accuracy requirements;

(2) the tank area to the production plant has a certain distance, in the northern region of the cold season steam accompanied by heat can not make the pipeline medium temperature uniformity, can not determine the representative temperature detection location, this situation flow measurement control program derived from the quality of the error is greater; process automation control process involves a number of control loops, mainly including raw material proportioning control loop, temperature control loop in the reaction process and finished products The process automation control involves several control loops, mainly including raw material proportioning control loop, temperature control loop during the reaction process and finished product measurement and packaging control loop. The project uses steam-heated reactor to complete the process reaction. The order of adding raw materials to the process is as follows: first add the liquid reaction materials to the reactor, then heat the medium temperature to slightly higher than the reaction temperature or the melting point of the solid reactants; slowly add the solid reactants under constant stirring, so that it can continue to react or dissolve to avoid precipitation. Due to the different formulations, each product requires about 4 liquid raw materials and 2 solid raw materials, and the dosing ratio is required to reach 0.1%.

Analysis of the original control program

The original control scheme of the Japanese company is 4 storage tanks feeding into the reactor through pumps, and the daily maintenance of the flowmeter requires more man-hours.

In order to accurately reflect the changes in quality, improve the program weighing sensor using a certain basis, the proposed quality measurement control program. That is, the weighing module monitoring system, the control of quality measurement is mainly realized through the weighing controller. As the weighing controller with fixed value comparison function, it can be directly used for automatic material sorting, screening, bulk quantitative packaging, production batching, filling, logistics in and out of the goods and other process control, has been widely used in the field of pharmaceuticals, raw material processing, petroleum, chemical industry and other fields.

Hardware components of weighing control system

Weighing Sensor

The company's high-end process FWC series weighing modules. The load cell consists of a pressurized head and a field junction box. The load cell is mounted on the side of the kettle and distributed into 120, the weight of the kettle is uniformly pressed on each load cell. 3 load cells enter into the field junction box in the form of MV, and the field junction box converts the signals of the 3 load cells into standard 4-20mA standard current signals and sends them to the weighing instrument.

Weighing Instrument

In order to better meet the requirements of the harsh industrial environment on site, the weighing instruments use PANTHER weighing terminals, which are equipped with the patented TraxDspTM digital filtering technology to obtain stable weight values in real time, and have a metal casing, which is safe for use in the reactor and meets the requirements for on-site installation. The weighing terminal allows the zero, full and tare functions to be done on site. In addition, the instrument is equipped with Profibus-DP fieldbus interface for high-speed communication with PLC in the control room to send the weight data to PLC for metering control.

Transfer pump and metering control valve

The conveying pump adopts airtight motor, and the start and stop of the conveying pump is automatically controlled by PLC. The metering control valve selects regulating valve, and the PLC sends out 4-20mA standard current signal to control the opening degree of the regulating valve. When the signal is 4mA, the valve is fully closed; when the signal is 20mA, the valve is fully open. The opening degree control design of the valve position adopts the adaptive control strategy according to the size of the feeding margin. When the feeding is close to the end, the valve position can be closed appropriately, and when the set quality is reached, it is all closed to ensure the accuracy of feeding.

Programmable controller (PLC)

Mitsubishi's Q series medium-sized PLC is used to complete the feeding and weighing control of the kettle. 4 kinds of PLC modules are used: switching input module, to receive on-site weighing start command, emergency stop command and pump open status; switching output module, to send out the start of the conveyor pump command and on-site metering instructions; analog output module, to regulate the metering valve opening; Profibus-DP communication module, which is mainly used in conjunction with the Mitsubishi Q series medium-sized PLC to ensure the feeding accuracy. -DP communication module, which communicates with the weighing instrument and collects the quality of the instrument.

Industrial computer

The industrial computer communicates with PLC through RS-232 port to complete the setting of weighing formula, monitoring the status of the whole dosage system, saving and printing the metering data, and realizing the automatic metering of feeding materials in the control room.

Software realization of weighing control system

PLC software programming PLC through the special programming software GXDeveloper on its programming, control logic with ladder diagrams written in the PLC for arithmetic control, time delay to open the feed pump.

The communication protocol is characterized by fast communication speed, strong anti-interference ability and error correction ability, and both Mitsubishi PLC and PANTHER weighing terminal have interfaces in accordance with Profibus-DP communication protocol.

Friendly interface programming

(1) The industrial computer exchanges data with PLC through the form of RS one 232 port, including the calculation to read the control status of field equipment in PLC, the process of feeding and measuring, fault alarm, etc.; the computer sends the control data to PLC, such as measuring quality, valve closing quality, pump closing quality.

(2) Field equipment status display. Field operation status adopts dynamic time tracking, can directly monitor the quality of the reactor, the operating status of the feed pump, metering control valve opening, has been feeding status, and so on.

(3) Formula editing. Workers can complete the new, modified, deleted, transferred to the formula and other operations, set the quality value of the measured material. Each recipe can include the name of the material and set the quality, that is, it is convenient for workers to set the quality and not easy to operate error.

(4) Measurement quality printing. When all the formulas are finished feeding, print all the set quality and the actual discharging quality.

(5) Data saving. It is convenient to check the history dosing record for tracking the production.

Conclusion

The project proposed weighing module monitoring system into the reactor feed control program, improve the operation of the entire process system capacity and level, improve the control system completely centralized operation capacity and reliability, to ensure the high quality of the product, the research and engineering applications have a strong real-time and stability, the production process control of similar chemical enterprises have a certain reference significance.